1. Field of the Invention
The present invention is directed to a process for reducing the level of acidic halide contamination in alkoxy silanes, particularly the level of chloride contamination in amino alkoxy silanes.
2. Description of Related Art
Siyl esters, also referred to as silyl ethers, i.e., organic silanes having alkoxy or aryloxy substituents, are typically prepared by reacting an organic silyl halide (halosilane) with an alcohol or phenol sometimes in the presence of an acidic acceptor to neutralize by-product acidic halide. Amino alkoxy silanes are conveniently prepared by reacting an alkoxy alkyl silyl halide having the formula (RO).sub.n R'.sub.3-n SiR"X where X is a halide and n is a number between 1 and 3, with a primary amine (R'"NH.sub.2) or a secondary amine (R"".sub.2 NH). R, R',R",R'" and R"" can be any of a wide variety of organic radicals including saturated or unsaturated aliphatic hydrocarbon radicals or aromatic radicals. Again, an acidic halide is produced as a by-product of the reaction.
In such systems, halide (e.g., chloride) contamination from by-product acidic halide is a reoccurring problem, particularly when the product is an aminosilane because a portion of the by-product acid, generally hydrogen chloride, unavoidably is neutralized by the amino moiety of the resulting aminosilane. This halide contamination of the silane product is undesired for a variety of reasons, including the potential for corrosion.
In the past, acidic halide contamination of alkoxy silanes generally, and particularly amino-substituted silanes, has been controlled by a post-reaction treatment with a strong base such as a metal alkoxide, e.g., sodium methoxide. Many of the commonly-used inorganic alkaline neutralizing agents, such as the alkali metal hydroxides, cannot be used, particularly in the case of alkoxy silanes, because water is produced by the neutralization reaction and contributes to silane product degradation via a hydrolysis mechanism. The same is true for sodium bicarbonate, which has been used for neutralizing certain organosilanes in the past. By-product water is a particularly troubling problem for higher molecular weight alkoxy silanes as it can lead to silane gelation.
Unfortunately, the quality of the silane product can also be adversely affected by the level of metal alkoxide addition. If an insufficient amount of the metal alkoxide is added, an undesirably high residual halide level is encountered in the silane product. On the other hand, the addition of even a small excess of the metal alkoxide commonly causes an unacceptably severe and irreversible color development in the alkoxy silane product, particularly those products having amine substitution. Such coloration is thought to be due to oxidation of the amine in the presence of the excess base.
Alkoxy silanes and amino-substituted alkoxy silanes in particular, find use in a variety of applications such as in laundry additives, and in caulking formulations, and as coupling agents between inorganic and organic surfaces such as a coupling agent for glass fiber. Normally these silane products have a very pale coloration. Thus, strong product coloration can be a significant problem confronting such uses.
In light of the above, great care must be exercised to obtain a proper neutralization end point when metal alkoxides are used for halide removal. This degree of care is very inconvenient and problematic in an industrial context. Use of metal alkoxy neutralization, therefore, tends to be very time-consuming and often leads to the unecomonical reworking, e.g., distillation, or in the extreme, discarding of over-neutralized products.
The present invention is directed to a new procedure for neutralizing alkoxy silanes, and amino-functional alkoxy silanes in particular, which essentially avoids the color-forming reactions encountered in the prior art procedure.